You could probably trim some more time off by disabling NetworkManager-wait-online.service. A few people have done this, including myself, without any noticeable effects.
Also if you don't use a modem you can do the same to ModemManager.service although that won't save you as much time. I have a few more than this disabled (like bluetooth) that I'm sure I don't need, but NetworkManager-wait-online.service is the big one.

I installed it some time ago but does the fact that it shows up here means that it gets started at start-up?

You can use systemctl list-unit-files --state enabled --no-page to list all enabled services and sockets, without a page-break (--no-page).
Change the enabled to disabled to see them or remove '--state enabled' to see all possible services.

I would expect that you could also disable any service that was installed by you without any system issues.

You also only need vboxdrv.service, the rest vbox* ones aren't really of any importance for common usage.
speech-dispatcher you can also get rid off if you don't need such abilities.
Same thing with dns-clean & pppd-dns if you are on a adsl connection...

You also only need vboxdrv.service, the rest vbox* ones aren't really of any use for common usage.
speech-dispatcher you can also get rid off if you don't need such abilities.
Same thing with dns-clean & pppd-dns if you are on a adsl connection...

I'm sure there are more you can disable, for example brltty.service is not needed unless you need braille display. The best thing is to google them.
Note that most of these only save a couple of tens of millisec's so not much saved.

By the way, what does your systemd-analyze critical-chain look like now that NetworkManager-wait-online.service is disabled?

brltty is a very good suggestion. As already mentioned above, speech-dispatcher if you don't use text-to-speech apps...and also, if you're always connecting via adsl or newer, dns-clean, pppd-dns and ModemManager as well.
To tweak even more than that, you could always google&read what each service does, why & how: however, by doing so, i can guarantee you that you'd probably gain more in the long-term...than having immediate results of 'shaving off' more time. Why? Because 4.631s (userspace) =>
if you spend your time tweaking this or that or the other one, you could probably save off 1 maximum 1.5 secs out of it.
Which in return raises the question: is there an actual difference that you could really feel between say 16.7secs and...18.007s?
If you had a 5400rpm hdd that took 51secs to boot, then dropping it down to 35secs makes sense...but 1 to 1.5 secs?
Try also checking if Fast Boot is enabled in your UEFI. It should be set to off when installing a distro, but it might help resulting in faster boot times afterwards (especially when it's not a cold boot).

Is there any service that shows up in each of these lists that shouldn't be there?

Define "shouldn't". Everything is there for a reason - it's not built incorrectly in the first place, it's not...Windows Millenium
Question is, if you also have a reason for this or that to be running: and that is something that only you can answer it.

Is there any service that shows up in each of these lists that shouldn't be there?

Define "shouldn't". Everything is there for a reason - it's not built incorrectly in the first place, it's not...Windows Millenium
Question is, if you also have a reason for this or that to be running: and that is something that only you can answer it.

I admit that it is a bit clumsily expressed. What I wanted to say was that if there is one or other services which should be on the other list and vice versa not that they shouldn't t be there at all.

I suppose that's one of the ironies of life, doing the wrong thing at the right moment -C.C.